Projection objective for a microlithographic projection exposure apparatus

ABSTRACT

A projection objective of a microlithographic projection exposure apparatus contains a plurality of optical elements arranged in N&gt;−2 successive sections A 1  to A N  of the projection objective which are separated from one another by pupil planes or intermediate image planes. According to the invention, in order to correct a wavefront deformation, at least two optical elements each have an optically active surface locally reprocessed aspherically. A first optical element is in this case arranged in one section A j , j=1 . . . N and a second optical element is arranged in another section A k , k=1 . . . N, the magnitude difference |k−j| being an odd number.

The invention relates to a projection objective for microlithographicprojection exposure apparatus, such as those used for the production ofmicrostructured components.

Microlithographic projection exposure apparatus, such as those used forthe production of large-scale integrated electrical circuits, have anillumination device which is used to generate a projection light beam.The projection light beam is aimed at a reticle which containsstructures to be imaged by the projection exposure system, and which isarranged in an object plane of a projection objective. The projectionobjective forms a reduced image of the structures of the reticle on aphotosensitive surface, which is located an image plane of theprojection objective and may for example be applied on a wafer.

Owing to the small size of the structures to be imaged, stringentrequirements are placed on the imaging properties of the projectionobjective. Imaging errors can therefore be tolerated only to a verysmall extent.

In general, imaging errors which occur are assigned to the followingcategories. On the one hand, there are imaging errors which result fromthe design of the projection objective, i.e. in particular thespecification of dimensions, materials and distances in the opticalelements contained in the projection objective. These design errors willnot be considered below.

On the other hand, there are imaging errors which are attributable toproduction or material errors and which it is generally sensible tocorrect only once the objective is finished. Examples of productionerrors include so-called form errors, which are intended to meandeviations from surface accuracy in the case of optical surfaces.Material errors, however, do not generally affect the condition of theoptically active surfaces, i.e. ones through which projection lightpasses, but lead to inhomogeneous refractive index profiles inside theoptical element. The possible causes of such imaging errors due toproduction or material will more generally be referred to below asperturbations, which may be locally very limited but which may evenextend over a sizeable region of the optical element in question.

In order to correct the wavefront deformations caused by suchperturbations, it is known to apply corrective structures to suitableoptical surfaces of the projection objective by means of reprocessing,which generally involves material erosion. The reprocessing gives thesurface an aspherical shape that is generally not rotationallysymmetric, and which differs from the shape on which the design of theprojection objective was based. Such reprocessing methods are describedat length in an article by C. Hofmann et al. entitled “NanometerAsphären: Wie herstellen und wofür?”, Feinwerktechnik und Meβtechnik 99(1991), 10, pp. 437 to 440.

The way in which the corrective structures required for the compensationmay be deduced from the wavefront deformations, which can generally berecorded by measuring techniques, is described at length in U.S. Pat.No. 6 268 903 B1. In the method described there, the optical elementwhose surface is to be locally reprocessed for perturbation compensationis preferably a plane-parallel plate, which is arranged between thereticle and the projection objective of the projection exposureapparatus. The known corrective element may furthermore be arrangedinside the objective, before or after a shutter contained therein, inwhich case a position where the projection light beam has a particularlysmall cross section is preferred.

However, it has been found that not all optically active surfaces insidethe projection objective are equally suitable for correcting a wavefrontdeformation by local reprocessing. Since projection objectives generallycontain a large number of optically active surfaces, it is not in factreadily possible to determine the correction potential computationallyfor all these surfaces. As a rule, therefore, empirical values or rulesof thumb are relied upon when choosing which optically active surfacesshould be reprocessed in order to compensate for perturbations. Forexample, a small diameter of the projection light beam is highlighted asan essential criterion in the method known from the aforementioned U.S.Pat. No. 6,268,903 B1.

It has been found, however, that the imaging properties often cannot beimproved sufficiently with the known criteria for the selection ofsurfaces to be reprocessed.

It is therefore an object of the invention to provide a projectionobjective in which more effective compensation for wavefrontdeformations that occur is achieved by reprocessing of optically activesurfaces.

This object is achieved by a projection objective of a microlithographicprojection exposure apparatus, having a plurality of optical elementsarranged in N≧2 successive sections A₁ to A_(N) of the projectionobjective which are separated from one another by pupil planes orintermediate image planes. In order to correct a wavefront deformation,at least two optical elements respectively have an optically activesurface locally reprocessed aspherically, a first optical element beingarranged in one section A_(j), j=1 . . . N and a second optical elementbeing arranged in another section A_(k), k=1 . . . N, and the magnitudedifference |k−j| being an odd number.

The invention is based on the discovery that the optically activesurfaces have fundamental differences in terms of their suitability forcorrecting wavefront deformations by appropriate reprocessing. Anessential criterion in this regard involves the sections of theprojection objective, separated from one another by pupil planes orintermediate image planes, where the surfaces being considered forreprocessing lie. In fact, it has been found that a wavefrontdeformation which is present in one section, and whose field dependencywith respect to the azimuth angle has an odd symmetry, can only becorrected by a reprocessed surface in the same section but not by areprocessed surface in a neighbouring section. Only with reprocessedsurfaces lying in the next but one section, or more generally in the n±2k^(th) section, is it possible to achieve almost complete correction.

The reason for this is that an image curvature occurs in shutter andintermediate image planes, which affects not only the odd components butalso the even components of the field dependency of a wavefrontdeformation caused by the perturbation.

If there was only a single perturbation at a known position in aprojection objective, then the wavefront deformation caused by it couldbe corrected by reprocessing an optically active surface which lies inthe same section as the perturbation. In general, however, there are amultiplicity of perturbations at unknown positions distributed over aplurality of sections of the projection objective. Their perturbingeffects are superimposed and deform the wavefront together.

For this reason, it is necessary to reprocess surfaces either in atleast two immediately adjacent sections, or in sections between whichthere are an even number of other sections. This means that for eachperturbation, a correction is carried out in the same section n, in thenext but one section n±2 or more generally in the section n±2 k, k=0, 1,2, . . . only this will ensure that any field-dependent wavefrontdeformation which is caused by the deformation, and which contains botheven and odd symmetry components, can be corrected almost completely.

If, as is generally desirable, it is also necessary to correct afield-independent wavefront error, i.e. one which is common to all thefield points, then a third optical element which is arranged in or closeto a pupil plane must also have a locally reprocessed optically activesurface. With a total of three reprocessed surfaces at positionsselected in this way inside the projection objective, it is thereforepossible to correct all wavefront deformations sufficiently well.

An exemplary embodiment of the invention will be explained below withreference to the drawing, in which:

FIG. 1 shows a projection objective according to the invention in asequential representation;

FIG. 2 shows a three-dimensional representation of an even wavefrontdeformation;

FIG. 3 shows a three-dimensional representation of an even wavefrontdeformation which is independent of the azimuth angle;

FIG. 4 shows a three-dimensional representation of an odd wavefrontdeformation.

FIG. 1 shows a meridian section through a projection objective, denotedoverall by 10, of a microlithographic projection exposure apparatus in asequential representation. As will be explained below, the sequentialrepresentation is more favourable here; a true representation of thisprojection objective, which also shows two plane mirrors used for thebeam deviation, can be found in the Applicant's WO 2004/010164 A2, thedisclosure of which is hereby fully incorporated.

The projection objective 10 is designed to image structures of a reticle(not shown in FIG. 1), which is introduced into an object plane 12 ofthe projection objective 10 during operation of the projection exposureapparatus, onto a photo-sensitive surface (also not represented). Thissurface may be applied on a wafer, for example, and it is arranged in animage plane 14 of the projection objective 10.

The projection objective 10 has a plane-parallel plate P arranged on theentry side and a multiplicity of lenses, of which only a few lenses L1to L8 and L13 and L19 that are more important for the explanation of theexemplary embodiment are provided with their own references in FIG. 1for the sake of clarity. The projection objective 10 furthermorecontains a spherical imaging mirror, denoted by S, through which lightcan be seen to pass in the sequential representation of FIG. 1.

The route of the projection light through the projection objective 10will be described below with reference to ray paths 15, which originatefrom two field points FP1 and FP2 arranged in the object plane 12. Afterpassing through the plane-parallel plate P, the projection light 15arrives at a plane deviating mirror (not shown in the sequentialrepresentation) and subsequently passes through the lenses L1 to L4.After reflection at the spherical imaging mirror S, which is arranged ina pupil plane E1, the projection light passes back through the lenses L1to L4 in the reverse order.

Since the illuminated field in the object plane 12 is arranged offsetwith respect to the optical axis 16 of the projection objective 10,after reflection at the imaging mirror S the light beam travels along aroute which leads to a spatial offset with respect to the incident lightbeam. Behind the lens L1, this offset between the light beam incident onthe imaging mirror S and the light beam reflected by it is so great thatthe reflected light beam does not strike the first deviating mirror butinstead, after passing through the lens L6, arrives at a second planedeviating mirror (which also cannot be seen in the sequentialrepresentation).

After reflection at the second deviating mirror, the projection lightpasses through a multiplicity of further lenses L7 to L19 and aplane-parallel closure plate, and finally arrives at the image plane 14of the projection objective 10.

There is an intermediate image plane E2 between the lenses L5 and L6,and a further pupil plane E3 in the lens L13. The two pupil planes E1and E3, as well as the intermediate image plane E2, subdivide theoverall projection objective 10 into a total of four sections A1, A2, A3and A4, the lenses L2, L3 and L4 being associated both with the sectionA1 and with the section A2 depending on the transmission direction ofthe projection light.

The pupil planes E1 and E3, as well as the intermediate image plane E2,respectively have the property that they turn over the image, i.e. theyinvert the imaging of the system. This will be explained in relation tothe pupil plane E1 with reference to the example of a primary ray 18originating from the field point FP1. In the section A1, i.e. beforereflection at the imaging mirror S, the primary ray 18 passes throughthe lens L2 below the optical axis 16, whereas after reflection at theimaging mirror S, it passes through the same lens L2 above the opticalaxis 16, i.e. point-reflected relative to the first case.

The inversion of the imaging through the pupil plane E1 can also be seenat the position of the intermediate image 20 produced in theintermediate image plane E2, which is turned over relative to the object(field points FP1 and FP2) in the object plane 12.

Similar considerations apply to the other pupil plane E3 and theintermediate image plane E2.

It will now be assumed that on its surface F5—these are numberedsequentially through the projection objective 10—the lens L2 has aperturbation indicated by 22 which, as explained, does not need to berotationally symmetric with respect to the optical axis 16. The cause ofthis perturbation may, for example, be a form error or a refractiveindex inhomogeneity of the lens material.

The effect of the perturbation 22 is that all light waves which passthrough the perturbation 22 are deformed in an undesirable way. In thiscontext, it should borne in mind that light waves originate from allfield points in the object plane 12. Whether the wavefront of one ofthese light waves will be deformed, and the way in which it is deformed,generally depends on the field point from which the relevant light waveoriginates. As a rule, specifically with perturbations outside a pupilplane, there are even field points whose light waves are not affected atall by the perturbation since they do not pass through the perturbation.

It is simplest to describe wavefront deformations in an exit pupil,since the ideal wavefront there is a spherical wave. The so-calledZernike polynomials Z_(r) are often employed to describe wavefrontdeformations, these being a function system usually represented in polarcoordinates, which are orthogonal in the unit circle. A wavefrontdeformation can then be described as a vector in an infinite-dimensionalvector space, the basis of which is spanned by the Zernike polynomials.In this context, the wavefront deformation is also referred to as beingexpanded in the Zernike polynomials Z_(r). The coefficients of thisexpansion are the components of the aforementioned vector.

If the position of the perturbation 22 is known, as will initially beassumed here, then it is possible to determine for the light wavesoriginating from each individual field point whether, and if so how, thewavefront of the respective light wave is deformed by the perturbation22. For example, the Zernike polynomials Z_(r) may be employed todescribe the wave-front deformation associated with a field point.Conversely, it is possible to determine therefrom how a particularwave-front deformation, defined by a single Zernike polynomial Z_(r), isdistributed over the individual field points. A tilt of the wavefront,as described for instance by the Zernike polynomial Z₂, may for examplebe commensurately stronger the further the field points are away fromthe optical axis.

The field dependency of the wavefront deformation caused by theperturbation may likewise be described by Zernike polynomials Z_(r). Forthis reason, the wavefront deformation is also referred to as beingexpanded in the field coordinates.

FIGS. 2 to 4 show a three-dimensional representation of the Zernikepolynomials Z₅, Z₉ and Z₈, respectively, which are given by

Z ₅(r,θ)=√{square root over (6)}r ²·cos(2θ),

Z ₉(r,θ)=√{square root over (5)}(6r ⁴−6r ²+1) and

Z ₈(r,θ)=√{square root over (8)}·(3r ²−2)·r·sin(θ).

The radial coordinate r in this case denotes the radial coordinate, i.e.the distance from the optical axis, and θ denotes the azimuth angle.

The Zernike polynomial Z₅(r,θ) represented in FIG. 2 is an even functionwith respect to point reflections on the optical axis. For such a pointreflection, which is described by the coordinate transformation

θ→θ+180° and

r→r

the following applies

Z _(r)(r,θ)=Z _(r)(r,θ+180°).

Since it is independent of the azimuth angle θ, the Zernike polynomialZ₉(r,θ) represented in FIG. 3 is likewise an even function with respectto point reflections.

The Zernike polynomial Z₈(r,θ) represented in FIG. 4, however, is an oddfunction with respect to such point reflections, since

Z ₈(r,θ)=−Z ₈(r,θ+180°).

If it is assumed here for the sake of simplicity that the fielddependency of a wavefront deformation can be described just by theZernike polynomial Z₅, which is even with respect to point reflections,then it can be seen that such a field dependency is unchanged by animage inversion associated with passing through a pupil plane orintermediate image plane E1, E3, or E2. Similar considerations apply tothe rotationally symmetric wavefront deformation represented in FIG. 3,since this is described by the Zernike polynomial Z₉ which is also even.

The situation, however, is different for a wavefront deformation whosefield dependency can be described just by an odd Zernike polynomial, forexample the Zernike polynomial Z₈ represented in FIG. 4. In this case,the effect of the image inversion at a pupil plane or intermediate imageplane E1, E3, or E2 is that the field dependencies are no longer thesame before and after such a plane.

If the perturbation 22 on the lens L2 is now to be compensated for byreprocessing an optically active surface on one of the other opticalelements, then the aforementioned different symmetry properties of thefield dependencies when the light waves pass through pupil planes orintermediate image planes E1, E2, E3 have wide-ranging consequences.This is because since the field dependencies of the wavefrontdeformations can generally be described only by a combination of evenand odd Zernike polynomials, the field-dependency components assigned tothese polynomials are transformed in different ways when passing throughpupil planes or intermediate image planes E1, E2, E3.

This means that in the section A2 next to the section A1 containing theperturbation 22, it is generally not possible to find an opticallyactive surface on optical elements contained therein with which the oddcomponents of the field dependencies can be eliminated by suitablereprocessing. Perturbation compensation on a single optically activesurface in this section A2 is successful only for the even components ofthe field dependency.

In the subsequent section A3, however, it is again possible tocompensate substantially for the perturbation 22 in the section A1 byreprocessing a single optically active surface, since the imageinversions caused by the pupil plane E1 and the intermediate image planeE2 balance each other out.

The different corrective potentials of optically active surfaces,according to which of the sections A1 to A4 contains the correspondingsurface, has been demonstrated with the aid of simulations. Table 1gives the residual error remaining as an RMS (root mean square value)and the corrective potential, indicated as a percentage, for suitablyreprocessed surfaces F_(i) in different sections A1 to A4. Thesimulation is in this case based on the assumption that the perturbation22 on the lens L2 can be described by the Zernike polynomial Z₁₃.

It can be seen clearly from the table that only the surfaces F6 and F24,respectively lying in the same section A1 and in the next but onesection A3, can substantially reduce the imaging errors due to theperturbation 22 by suitable reprocessing, and therefore have a largecorrective potential. The surfaces F16 and F46 which lie in the sectionsA2 and A4, respectively, and which would likewise be considered forconventional corrective surface configuration, have only a very smallcorrective potential compared with these.

TABLE 1 Corrective potential of different correction surfacesREPROCESSED ERROR CORRECTIVE SURFACE SECTION (RMS) POTENTIAL no — 1.6  0% reprocessing F6 A1 0.1 95.7% F16 A2 1.3 15.8% F24 A3 0.2 86.1% F46A4 1.3 21.7%

In the preceding discussion, it was assumed that there is only oneperturbation 22 whose position is known. However, a wavefrontdeformation is contributed to in general by a plurality ofperturbations, which furthermore cannot be located, or at least not withtolerable outlay. This means that it is not generally possible correct awavefront deformation by a single reprocessed surface. If one surface inthe section A1 and another surface in the section A3 were reprocessed,for example, then generally no contributions to the wavefrontdeformation which are attributable to perturbations in the section A2lying in between could therefore be corrected.

It follows from this argument that an effective correction of unknownperturbations can generally be carried out only by reprocessing surfacesin at least two sections which are consecutive, or between which thereare an even number of other sections. In the present exemplaryembodiment, these could be the section combinations A1 and A2, A2 andA3, A3 and A4, A1 and A4.

With the measures described above, it is only possible to correctcomponents of a wavefront deformation which vary from field point tofield point. A field-independent component (offset) of a wavefrontdeformation, i.e. one which is common to all the field points, cannot becorrected by local reprocessing of surfaces in the sections A1 to A4,but only by reprocessing of surfaces which lie in or close to one of thepupil planes E1, E3. Since wavefront deformations often have such anoffset component as well, not just two but three surfaces will have tobe reprocessed in order to be able to at least approximately correctwavefront deformations of the most general type.

1. Projection objective of a microlithographic projection exposure apparatus, comprising a plurality of optical elements (P, S, L1 to L8, L13, L19) arranged in N≧2 successive sections Ai to A_(N) of the projection objective (10) which are separated from one another by pupil planes (E1, E3) or intermediate image planes (E2), characterised in that in order to correct a wavefront deformation, at least two optical elements respectively have an optically active surface locally reprocessed aspherically, a first optical element being arranged in one section A_(j), j=1 . . . N and a second optical element being arranged in another section A_(k), k=1 . . . N, and the magnitude difference |k−j| being an odd number. 2.-6. (canceled) 